The present invention relates to a mechanical actuating device for a spot-type disc brake comprising a brake caliper embracing a brake disc inwardly or outwardly, the brake caliper including at least one axial cylindrical bore for accommodating a substantially cylindrical hollow brake piston which acts on one brake shoe via an intermediate plate, if necessary, an actuating spindle arranged axially within the brake piston and adapted to be turned around its axis by means of an actuating means, the actuating spindle cooperating with an actuating nut likewise disposed in the brake piston and abutting with its outer surface at least partly the inner wall of the brake piston such that the brake piston is advanced upon a relative axial displacement between the actuating spindle and the actuating nut caused by the actuating spindle being turned, and an automatic adjusting device for the brake piston likewise disposed in the brake piston. The present invention refers in particular to a special design of a floating-caliper brake, i.e., to the so-called fist-type caliper brake, in which one single bridge embraces the brake disc in a fist-like manner.
In a known spot-type disc brake adapted to be actuated both hydraulically and mechanically, for example German Patent DE-AS No. 1,575,996, the brake piston is located axially freely movable on the actuating nut and, for actuation thereof, is acted upon from the inside by an adjusting spindle cooperating with the adjusting device. The known brake is constructed of a comparatively great number of components, and the actuating force is exerted on an end of the brake piston through a relatively small surface of the adjusting spindle. For the resetting of the adjusting device prior to a pad replacement due to pad wear, it is required to unscrew a screw cap and to take out a wedge, which is problematic in practice because of the danger of the wedge being jammed by corrosion, for example, and because such a small component gets easily lost. Construction and assembly of the known brake are also rather costly.